1998 Yamaha C 150

[MEM1971]

I have a 1998 Yamaha C150 I bought brand new may of 98. It has been an excellent motor and will be hard not to go back with Yamaha when I finally upgrade. Only thing to ever be done to this motor is change water pump impeller, lower unit fluid, and spark plugs. Recently I have been getting high temp alarm sometimes shutting down cylinders. Once I bring it back down to idle after a minute or so it goes off and all is good. Water is coming out great and last time it went off I hurried and jumped back there to feel the water and it was cool. A little warm, Normal. It only does it trimmed up and almost wide open. Once did it pulling kids on a tube and last time it did it I was not trimmed all the way up or wide open but I had a lot of ice, tackle, another person on board. I truly do not think its actually getting hot but it has all the symptoms. I don't run wide open anymore but I got caught in the worst storm of my life a few weeks ago and was following someone back to launch in the rain and dark after waiting on lightning to stop for over 3 hours. Gauges are not working yet but do have a rpm gauge to put in it. Just guessing id say somewhere around 40-45 mph is where it starts. Prop needs fixing but boat used to run 55mph. I do not believe this motor has a computer and I add my own oil to the gas , 2 stoke, which is something I wanted back then when a lot were having trouble with their oil pumps going out. Are there sensors or thermostats that can go bad? I have a new water pump impeller im putting in this weekend but since its stream is great I doubt this is the cause. Anyone out there with knowledge of this particular motor I would sure appreciate some guidance.